Teacher Notes

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Step 1: Determine Q Factor

To determine the Q factor of the active narrow band-pass filter circuit, first construct the circuit in MultiSim.

Using the Bode Plot function on Multisim, we found the positive and negative 3dB values and the bandwidth.

From the Bode plot, the resonant frequency was 331.131 Hz which gives us the frequency for the note e.

With this values, we could then determine the Q factor using the following equation:

Q-factor = CenterFrequency / Bandwidth

With this, we found our Q-factor to be 10.

Step 2: Things You Will Need

- Bread board for initial construction (after construction you can then solder onto a PCB board)

- Electric Guitar

- Wires

- Digital Multimeter

- 6.3mm Stero Jack sockets (switched)

- 8 Ohm Loudspeaker

- 6V Battery Souce or a 12V DC source

- (2) 502 Potentiometers

- 102 Potentiometer

- 104 Potentiometer

- 2uf capacitor

- 27g operational amplifier

* If time permits we are going to try a TDA 7052 with 10 uf capacitor, 503 potentiometer, 100uf capacitor, 220uf capacitor, and potentiometer for volume control *

Step 3: Construct Circuit

Draw out a simplified circuit, as shown in the diagram. Build the circuit using the components listed in Step 2. In the trial run, put the operational amplifier, resistors, and capacitors onto the breadboard and connect them with wires. If the circuit works, the parts can be soldered onto a PCB board.

Step 4: Test Amplifier

Connect the guitar jack into the circuit via leads, then plug in the guitar. Strum note "little E" and listen to it through the speaker!

Step 5: Troubleshooting

In case the amplifier does not work, change elements of the circuit. Some of the problems we ran into include:

1. The signals produced by the guitar were not strong enough to be detected by the amplifier. To solve this problem, we changed the 741 operational amplifier we had initially to an LM386.

2. When we added a band pass filter to filter out our single note, it filtered out every note and no sound came through the speaker. Instead we needed to connect two OP27G op-amps, one to the input end of our circuit and the other to the output end. The two new op-amps allowed us to help filter the frequencies better. The OP27G op-amp placed at the beginning of the circuit was designed to help amplify the input signal. The one added to the end was designed with a low gain to help filter out the extra noise in the circuit.

After implementing these changes, our guitar amplifier worked and produced a cleaner sound with less noise.

Step 7: Final Design

After trying out several changes to our amplifier design, we were able to improve our amplifier. We wanted a low gain on the front end of the amplifier, followed by two cascaded band pass filters. We then added a LM386 to give variable gain to the speaker. To control the volume, a potentiometer can be added between the speaker and the output of the amplifier. The circuit diagram and Bode plot of our improved amplifier is provided here, as well as a video of the amplifier at work.